It looks like a model 125 "RE" with some modifications. I don't know enough about the REs to give an accurate date of manufacture. If I had to guess I'd say it is a late model sometime in the early 1960s or late 50s.

The Royal Enfield Owners Club in the UK might have the information you are looking for, or try Hitchcock's in the UK.

I'm much more familiar with the twin Royal Enfield and they have the numbers on the left side of the engine case just below the cylinder base and on the front of the case for the 1956 & earlier models. On the small singles I have no idea. The number you posted looks like an engine number from that era. Its possible they only had one number for both engine & frame on the little singles.